Carol:
The use of pure tin lead finish on solid state products for military
applications is currently prohibited due to tin whisker problems. In fact as I
understand it a recent satellite failure was traced to the use of pure tin on a
relay.
Apparently the use of lead within tin inhibits the growth of tin whiskers. Do
the other materials within the new alloys (e.g Copper) also inhibits tin
whiskers?
Also how many of these materials can be plated?
Pete Brooks
-----Original Message-----
From: Carol Handwerker [SMTP:[log in to unmask]]
Sent: Thursday, July 08, 1999 12:17 PM
To: [log in to unmask]
Subject: [LF] Background information: reliability testing done by
the NCMS project
NCMS tested seven lead-free solders and eutectic Pb-Sn -
Solder - Liquidus Temperature
Sn-37Pb - 183°C
Sn-3.5Ag - 221°C
Sn-58Bi - 139°C
Sn-3Ag-2Bi - 220°C
Sn-2.6Ag-0.8Cu-0.5Sb - 211°C (CASTIN)
Sn-3.4Ag-4.8Bi - 210°C
Sn-2.8Ag-20In - 187°C
Sn-3.5Ag-0.5Cu-1Zn - 221°C
Among the reliability tests were thermal cycling of surface mount
reliability test vehicles (RTV-SM) tested for:
6673 cycles at 0°C/100°C
5000 cycles at -55°C/125°C
with
84 J-leaded PLCC
132 gull-wing leaded BQFP
1206 discrete chip resistors - alumina body
1206 discrete chip capacitors - barium titanate body
44 castellated I/O LCCC (for getting early failure information for
modeling
only)
on multilayer FR-4 epoxy boards.
The components on the RTV-SM represent a variety of commonly used
surface
mount components. Component choice was based primarily on the expected
stress/strain response during thermal cycling as well as the lead shape,
body size, and availability in a daisy chain configuration. Components
were
obtained either with a 100% Sn finish on the component leads, or with a
Sn/Pb finish that was stripped and refinished by a commercially
available
Sn-dipping process.
Components were connected daisy chained. Performance was monitored
electrically and by cross-sections. Double-sided assemblies were used
for
electrical testing to maximize sample size, while single-sided
assemblies
were used for cross sectioning to facilitate sample preparation.
Multilayer FR-4 epoxy glass boards were chosen for the RTV-SM to match
product requirements. Components were distributed across the board so
that
each component type occupied both edge and central positions. Components
placed near the edge of the boards often fail earlier than those located
more centrally on the board. A commercially available immersion Sn
finish
was used because it exhibited the best solderability in manufacturing
trials.
Copies of the NCMS report with the test results and the CD-ROM with all
project information can be obtained at
http://www.ncms.org/3portfolio/1ProjectPortfolio/pubs.htm
At 03:49 PM 7/7/99 EDT, you wrote:
>In a message dated 7/7/99 9:35:30, [log in to unmask] writes:
>>Hi, Werner,
>>What tests are sufficient to ensure "reasonable" reliability [of
solder
>joints]?
>>Carol
>
>Hi Carol ,
>This sure is a loaded question, and can not be answered by a brief
statement
>without being flip-so, you asked for it!
>Personally, and from experience, I rather assure solder joint
reliability
>using an adequate modeling approach, taking into account both the
>physics-of-failure and the statistical failure distribution. The reason
is
>that it is no more error-prone than testing and much less expensive and
>time-consuming. The time and cost factors of testing has led to test
>short-cut, with sometimes catastrophic consequences.
>Using a 'Design for Reliability (DfR)'-approach, of course, requires a
>reasonably good model. For near-eutectic Sn/Pb solders and their
derivatives,
>we have the data on which a number of modeling approaches used in the
>industry are based. But for many of the newer soldering alloys,
including
all
>of the lead-free solders, we do not have sufficient data to determine a
>fatigue reliability model. Thus, until such time, any modeling requires
>assuming that the solder in question behaves similar to Sn/Pb and
multiplying
>some safety (actually ignorance) factor (>=2xlife to acceptable failure
>probability).
>For the new solders, side-by side cyclic testing with eutectic (or
60/40)
>Sn/Pb solder from -20<->+100C (125C if substrate glass transition
temperature
>is at least 150C) with 15 minute dwells at each temperature extreme (24
>cycles/day) with conventional chambers (or 5 minute dwells with
chambers
>modified to provide temperature uniformity in the whole test volume;
~100
>cycles/day) with at least 32 equal continuity daisy-chains monitored
with an
>Anatech Event Detector for both solders, is necessary. This will give
an
>adequate data base to draw valid comparisons with near-eutectic Sn/Pb
>solders, and, to me more importantly, from which a fatigue reliability
model
>can be derived.
>Tests that are sufficient to ensure "reasonable" reliability will be
totally
>dependent on the product design together with its application. But to
design
>such a test, you have to have an appropriate fatigue reliability model
from
>which a valid acceleration factor for the test vis-a-vis the use
conditions
>can be determined. Electronic application vary from easily met
reliability
>reqirements for consumer products (~1,000 cycles (3 yrs) @ delta-T's
~10C &
>10% acceltable failure probability) to difficult to meet requirements
for
>low-earth-orbit satellites (~90,000 cycles (10 yrs) @delta-T's ~35C &
0.001%)
>or automotive-under-hood equipment (~10,000 cycles (~3 yrs) @ delta-T's
~100C
>& 0.1%).
>
>Werner Engelmaier
>Engelmaier Associates, L.C.
>Electronic Packaging, Interconnection and Reliability Consulting
>7 Jasmine Run
>Ormond Beach, FL 32174 USA
>Phone: 904-437-8747, Fax: 904-437-8737
>E-mail: [log in to unmask], Website: www.engelmaier.com
>
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>Please visit IPC's Center for Lead-Free Electronics Assembly
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**************************************
Carol A. Handwerker
Chief, Metallurgy Division
NIST
100 Bureau Drive Stop 8550
Gaithersburg MD 20899-8550
Office:(301) 975-6158
Fax:(301) 975-4553
e-mail:[log in to unmask]
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IPCWorks -October 25-28 featuring an International Summit on Lead-Free
Electronic
Assemblies.
Please visit IPC's Center for Lead-Free Electronics Assembly
(http://www.ipc.org/html/leadfree.htm ) for additional information.
For technical support contact Gayatri Sardeshpande [log in to unmask] or
847-790-5365.
################################################################
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Leadfree E-Mail Forum provided as a free service by IPC using LISTSERV 1.8c
################################################################
To subscribe/unsubscribe, send a message to [log in to unmask]
with following text in the body:
To subscribe: SUBSCRIBE Leadfree <your full name>
To unsubscribe: SIGNOFF Leadfree
################################################################
IPCWorks -October 25-28 featuring an International Summit on Lead-Free Electronic
Assemblies.
Please visit IPC's Center for Lead-Free Electronics Assembly
(http://www.ipc.org/html/leadfree.htm ) for additional information.
For technical support contact Gayatri Sardeshpande [log in to unmask] or 847-790-5365.
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